The present invention relates to rackets for playing tennis, squash, badminton, racketball or other games.
Coventional games rackets which are currently used and are commercially available have their playing surfaces formed by opposite sides of interwoven stringing lying in a single median or central plane of the head frame of the racket.
It has previously been proposed to replace the single, central set of interwoven stringing by two sets disposed in generally parallel planes spaced apart by a distance approximating the thickness of the head frame. Whilst such a "double-strung" racket construction possesses significant advantages over "centrally-strung" rackets, it also presents specific problems which are believed to account for the fact that such double-strung rackets have not previously been produced on a commercial scale.
A major problem occurs in achieving the required precise overall weight of the racket, weight of the racket head, and balance. These factors are all critical, and, for example, modern tournament tennis players in general favour tennis rackets weighing of the order of 13 to 131/2 ounces or less, the weight and weight distribution being critical within narrow limits amounting to small fractions of an ounce. However, since the head frame of a double-strung racket carries two sets of tensioned stringing it must be able to withstand approximately twice the inward loading created by the tension of the stringing of a conventional centrally-strung racket. This requires the head frame to be strengthened, and this has in the past necessitated increasing the amount of material in the head frame, thereby unacceptably increasing the head weight and unbalancing the racket by making it head heavy. This is aggrevated by the extra weight contributed by the additional set of stringing which can, for example add to the head weight by approximately 1/2 ounce.
This problem is substantially reduced by the double-strung racket constructions disclosed in my U.S. Pat. No: 4,049,269, to which reference should be made. For example, in the embodiment described with reference to FIGS. 4 to 8, the stringing is located by, and passes through apertures which extend transversely between opposite side surfaces of the head frame, the apertures being formed in a strip which is carried by, overlies, and extends circumferentially around, the outer peripheral surface of the main structural part of the head frame. Thus, the structural frame part is not weakened by the provision of stringing-receiving apertures or grooves, and the strip, which forms part of the head frame, may also reinforce the structural frame part.
Whilst the rackets disclosed in my aforesaid Specification are particularly advantageous, these rackets, as well as previously proposed double-strung rackets, give rise to other problems. One of these concerns the "tying-off" or "knotting off" of the ends of the stringing, which ends number at least four and possibly upto eight. It has been found to be undesirable to "knot-off" on the opposite side surfaces of the frame, since this is unsightly, and it also renders the playing surfaces uneven. In addition, the knots block the associated stringing-receiving apertures, rendering it difficult or impossible to pass further portions of the stringing therethrough, either during initial stringing or when partial re-stringing is required.
Another problem concerns the appearance of the stringing. From a stress and an aesthetic viewpoint, the portions of the stringing in one plane or set should preferably extend parallel to the opposing portions of the stringing in the other plane or set. Whilst this can be achieved in one of two basic ways with the rackets disclosed in my aforesaid Specification, both possess disadvantages. Firstly, the stringing-receiving apertures can be inclined so that they are not perpendicular to the planes of the opposite side surfaces of the head frame, but this considerably complicates production of the apertures. Secondly, stringing sequences or systems can be employed which involve passing the stringing circumferentially along one or both opposite side surface between adjacent perpendicular apertures. However, this subjects the opposite side surfaces of the frame to uneven circumferential tensions tending to warp the frame. In addition, the stringing has to pass an increased number of times through at least some of the individual apertures, and in particular at least four times through four or five apertures, requiring the internal diameter of those apertures to be excessively increased. Neither of these two systems solve the previously mentioned "knotting-off" problem.
Further problems occur in obtaining the required stiffness of the frame, since the stresses imposed on the frame of a double-strung racket are not only greater than, but also different from, those imposed on a centrally-strung racket. Firstly, when a ball stikes one of the playing surfaces of a double-strung racket, the resultant deflection of that playing surface inwardly towards the other playing surface tends to roll or twist the frame inwardly. This tendency is greater with a double-strung racket, since the deflected playing surface is effectively attached to the associated side surface of the frame, not to the centre of the inner peripheral surface of the frame. Although the tension in the stringing portions of the other playing surface opposes this rolling or twisting tendency, the stringing portions are resilient, and the frame itself should therefore possess superior roll or torsional stiffness to resist this tendency.
Secondly, it is one of the principle advantages of a double-strung racket that a ball may strike the marginal zones of its playing surface, over or adjacent the frame, without being uncontrollably deflected as in a "wood" shot with a centrally-strung racket. However, with such an off-centre shot, particularly where the ball strikes the toe or tip of the racket opposite the handle, the power of the return shot will be reduced and the racket will, to the player, feel slightly "dead", if the frame is relatively weak or flexible in bend. Therefore, in order to achieve the required good response and feel to such off-centre strokes, the frame must possess superior stiffness in bend.
It is an object of the present invention to provide a frame construction for a double-strung racket which enables at least some of the previously mentioned problems to be overcome or substantially alleviated.